1. Field of the Invention
The present invention relates to a clamping device for clamping a die set during injection molding operation and an injection molding machine provided with the clamping device.
2. Description of the Related Art
A clamping device of an injection molding machine includes a stationary platen to which a stationary die is attached, a movable platen to which a movable die is attached, a pressure mechanism that generates a clamping force, etc. There are clamping devices of various types. For example, a toggle-type clamping device is described in Jpn. Pat. Appln. KOKAI Publication No. 11-170322, and a composite clamping device in Jpn. Pat. Appln. KOKAI Publication No. 2001-1381. These clamping devices each include four tie bars that are arranged at the respective outer peripheral portions of the stationary and movable platens.
The tie bars are located individually at the four corners of each of the stationary and movable platens. The clamping force is transmitted mainly from a central part of each platen to the die set. Depending on the stiffness of the stationary and movable platens, therefore, the outer peripheral portions of these platens may possibly be deformed. As in the clamping device shown in FIG. 9, for example, the respective outer peripheral portions of a stationary platen 1 and a movable platen 2 are deformed toward each other by a tension that acts on tie bars 3.
A stationary die 4 and a movable die 5 are attached to the stationary platen 1 and the movable platen 2, respectively. The stationary and movable dies 4 and 5 constitute a die set 6. If the platens 1 and 2 are deformed, therefore, a force indicated by arrow P1 in FIG. 9 acts on the stationary and movable dies 4 and 5. The force indicated by arrow P1 causes a leakage of a material such as a resin injected into the die set 6 through parting surfaces 7 of the die set. The smaller the die dimensions, the more noticeable this trend is.
Deformation of the die set can be prevented by increasing the stiffness of the platens. In order to increase the stiffness of the platens, however, their strength should inevitably be enhanced by increasing their thicknesses. In this case, there is a problem that the clamping device is increased in size or weight.
In the clamping device described in Jpn. Pat. Appln. KOKAI Publication No. 2001-1381, convex load transmission portions are formed on the die side of platens in order to prevent deformation of a die set. Die fixing plates for use as intermediate members are arranged individually between the die set and the load transmission portions. In one such conventional clamping device, as shown in FIG. 10, a clamping force may possibly be transmitted mainly from a central part of each of platens 1 and 2 to a die set 6 during clamping operation. This clamping force causes die fixing plates 8a and 8b to push the die set 6 in the direction of arrow P2 in FIG. 10. Thus, deflection of the platens 1 and 2 can be prevented from being transmitted to the die set 6, so that parting surfaces 7 can be prevented from being separated from each other.
In the conventional clamping device shown in FIG. 11, the die fixing plates are arranged individually between a die set and platens. If the die dimensions are larger than the dimensions of pressure surfaces of load transmission portions, therefore, a clamping force acts mainly on a central part of a die set 6. Accordingly, the die fixing plates are deformed so that parting surfaces 7 at the outer peripheral portion of the die set 6 easily open. Thus, if a cavity is formed near the outer peripheral portion of the die set 6, a material that is fed into the cavity may possibly leak through the parting surfaces 7.